CH domains revisited

Xin repeats define a novel actin-binding motif

Xin is a protein that is expressed during early developmental stages of cardiac and skeletal muscles. Immunolocalization studies indicated a peripheral localization in embryonic mouse heart, where Xin localizes with beta-catenin and N-cadherin. In adult tissues, Xin is found primarily in the intercalated discs of cardiomyocytes and the myotendinous junctions of skeletal muscle cells, both specialized attachment sites of the myofibrillar ends to the sarcolemma. A large part of the Xin protein consists of unique 16 amino acid repeats with unknown function. We have investigated the characteristics of the Xin repeats by transfection experiments and actin-binding assays and ascertained that, upon expression in cultured cells, these repeats bind to and stabilize the actin-based cytoskeleton. In vitro co-sedimentation assays with skeletal muscle actin indicated that they not only directly bind actin filaments, but also have the capability of arranging microfilaments into networks that sediment upon low-speed centrifugation. Very similar repeats were also found in 'Xin-repeat protein 2' (XIRP2), a novel protein that seems to be expressed mainly in striated muscles. Human XIRP2 contains 28 Xin repeats with properties identical to those of Xin. We conclude that the Xin repeats define a novel, repetitive actin-binding motif present in at least two different muscle proteins. These Xin-repeat proteins therefore constitute the first two members of a novel family of actin-binding proteins.

Actin filament binding by a monomeric IQGAP1 fragment with a single calponin homology domain

IQGAP1 is a homodimeric protein that reversibly associates with F-actin, calmodulin, activated Cdc42 and Rac1, CLIP-170, beta-catenin, and E-cadherin. Its F-actin binding site includes a calponin homology domain (CHD) located near the N-terminal of each subunit. Prior studies have implied that medium- to high-affinity F-actin binding (5-50 microM K(d)) requires multiple CHDs located either on an individual polypeptide or on distinct subunits of a multimeric protein. For IQGAP1, a series of six tandem IQGAP coiled-coil repeats (IRs) located past the C-terminal of the CHD of each subunit support protein dimerization and, by extension, the IRs or an undefined subset of them were thought to be essential for F-actin binding mediated by its CHDs. Here we describe efforts to determine the minimal region of IQGAP1 capable of binding F-actin. Several truncation mutants of IQGAP1, which contain progressive deletions of the IRs and CHD, were assayed for F-actin binding in vitro. Fragments that contain both the CHD and at least one IR could bind F-actin and, as expected, removal of all six IRs and the CHD abolished binding. Unexpectedly, a fragment called IQGAP1(2-210), which contains the CHD, but lacks IRs, could bind actin filaments. IQGAP1(2-210) was found to be monomeric, to bind F-actin with a K(d) of approximately 47 microM, to saturate F-actin at a molar ratio of one IQGAP1(2-210) per actin monomer, and to co-localize with cortical actin filaments when expressed by transfection in cultured cells. These collective results identify the first known example of high-affinity actin filament binding mediated by a single CHD.

The nucleotide exchange factor Cdc24p may be regulated by auto-inhibition

Site-specific activation of the Rho-type GTPase Cdc42p by its guanine-nucleotide exchange factor (GEF) Cdc24p is critical for the establishment of cell polarity. Here we show that binding of Cdc24p to the small GTPase Rsr1p/Bud1p is required for its recruitment to the incipient bud site. Rsr1p/Bud1p binds to the CH-domain of Cdc24p, which is essential for its function in vivo. We have identified a cdc24-mutant allele, which is specifically defective for bud-site selection. Our results suggest that Cdc24p is auto-inhibited by an intramolecular interaction with its carboxy-terminal PB1-domain. Rsr1p/Bud1p appears to activate the GEF activity of Cdc24p in vivo, possibly by triggering a conformational change that dissociates the PB1-domain from its intramolecular binding site. Genetic experiments suggest that Bem1p functions as a positive regulator of Cdc24p by binding to the PB1-domain of Cdc24p, thereby preventing its re-binding to the intramolecular inhibitory site. Taken together, our results support a two-step molecular mechanism for the site-specific activation of Cdc24p, which involves Rsr1p/Bud1p and the adaptor protein Bem1p.

Structural and functional analysis of the actin binding domain of plectin suggests alternative mechanisms for binding to F-actin and integrin beta4

Plectin is a widely expressed cytoskeletal linker. Here we report the crystal structure of the actin binding domain of plectin and show that this region is sufficient for interaction with F-actin or the cytoplasmic region of integrin alpha6beta4. The structure is formed by two calponin homology domains arranged in a closed conformation. We show that binding to F-actin induces a conformational change in plectin that is inhibited by an engineered interdomain disulfide bridge. A two-step induced fit mechanism involving binding and subsequent domain rearrangement is proposed. In contrast, interaction with integrin alpha6beta4 occurs in a closed conformation. Competitive binding of plectin to F-actin and integrin alpha6beta4 may rely on the observed alternative binding mechanisms and involve both allosteric and steric factors.

An atomic model for actin binding by the CH domains and spectrin-repeat modules of utrophin and dystrophin

Utrophin and dystrophin link cytoskeletal F-actin filaments to the plasmalemma. Genetic strategies to replace defective dystrophin with utrophin in individuals with muscular dystrophy requires full characterization of these proteins. Both contain homologous N-terminal actin-binding motifs composed of a pair of calponin-homology (CH) domains (CH1 and CH2) that are connected by spectrin-repeat modules to C-terminal membrane-binding sequences. Here, electron microscopy and 3D reconstruction of F-actin decorated with utrophin and dystrophin actin-binding constructs were performed using Utr261 (utrophin's CH domain pair), Utr416 (utrophin's CH domains and first spectrin-repeat) and Dys246 (dystrophin's CH domain pair). The lozenge-like utrophin CH domain densities localized to the upper surface of actin subdomain 1 and extended azimuthally over subdomain 2 toward subdomains 3 and 4. The cylinder-shaped spectrin-repeat was located at the end of the CH domain pair and was aligned longitudinally along the cleft between inner and outer actin domains, where tropomyosin is present when on thin filaments. The connection between the spectrin-repeat module and the CH domains defined the orientation of CH1 and CH2 on actin. Resolution of utrophin's CH domains and spectrin-repeats permitted docking of crystal structures into respective EM densities, leading to an atomic model where both CH and spectrin-domains bind actin. The CH domain-actin interaction for dystrophin was found to be more complex than for utrophin. Binding assays showed that Utr261 and Utr416 interacted with F-actin as monomers, whereas Dys246 appeared to associate as a dimer, consistent with a bilobed Dys246 structure observed on F-actin in electron microscope reconstructions. One of the lobes was similar in shape, position and orientation to the monomeric CH domains of Utr261, while the other lobe apparently represented a second set of CH domains in the dimeric Dys246. The extensive contact made by dystrophin on actin may be used in vivo to help muscles dissipate mechanical stress from the contractile apparatus to the extracellular matrix.

Expression of calmin, a novel developmentally regulated brain protein with calponin-homology domains

We examined the expression in the mouse brain of a recently isolated protein named calmin that has two calponin-homology domains in tandem at the N-terminus and a transmembrane domain at the C-terminus. Calmin mRNA and protein were detected in neurons of the hippocampus, cerebral cortex, and thalamus, Purkinje cells, and also in the choroid plexus and ependymal cells. The protein is present predominantly in dendrites and cell bodies of the neurons, but not in axons. Furthermore, the amounts of calmin mRNA and protein increase during the period of maturation of the mouse brain after birth, in a manner similar to that of PSD95 and synaptophysin. These results indicate that calmin may be involved in the development and/or maintenance of neuronal functions.

Plectin-isoform-specific rescue of hemidesmosomal defects in plectin (-/-) keratinocytes

The various plectin isoforms are among the major crosslinking elements of the cytoskeleton. The importance of plectin in epithelia is convincingly supported by the severe skin blistering observed in plectin-deficient humans and mice. Here, we identified plectin 1a (> 500 kDa), a full length plectin variant containing the sequence encoded by the alternative first exon 1a, as the isoform most prominently expressed in human and mouse keratinocytes. In skin sections and cultured keratinocytes, plectin 1a was shown to colocalize with hemidesmosomal structures. In contrast, a second isoform expressed in epithelia, plectin 1c, differing from 1a merely by a short N-terminal sequence, colocalized with microtubules. Expression of plectin 1a, but not of its N-terminal fragment alone, or of a third alternative full length isoform (plectin 1), restored the reduced number of hemidesmosome-like stable anchoring contacts in cultured plectin-null keratinocytes. Our results show for the first time that different isoforms of a cytolinker protein expressed in one cell type perform distinct functions. Moreover, the identification of plectin 1a as the isoform defects in which cause skin blistering in plectin-related genetic diseases, such as epidermolysis bullosa simplex MD and epidermolysis bullosa simplex Ogna, could have implications for the future development of clinical therapies for patients.

Adaptor protein APS binds the NH2-terminal autoinhibitory domain of guanine nucleotide exchange factor Vav3 and augments its activity

The N-terminal calponin homology (CH) domain of Vav guanine nucleotide exchange factor is thought to serve a regulatory role in the autoinhibition, however, its precise function is not entirely clear. We found that the adaptor molecule APS could bind the CH domain of Vav3, a member of the vav proto-oncogene family. The binding of Vav3 and APS was apparently stabilized by the tyrosine phosphorylation of Vav3 by Lck, and the association of APS with Vav3 in turn enhanced the Lck-mediated phosphorylation of Vav3. Focus formation assays demonstrated that APS could increase the transforming activity of proto-Vav3. Further analyses revealed that the Vav3 CH domain could bind the pleckstrin homology (PH) domain of APS and that this binding was indispensable to enhance the transforming activity of Vav3. We present here a novel stimulatory mechanism of Vav3 in which APS directly relieves the autoinhibitory CH domain and furthermore enhances its tyrosine phosphorylation by Lck.

Isolation of HELAD1, a novel human helicase gene up-regulated in colorectal carcinomas

To investigate the mechanisms of colorectal carcinogenesis, we searched for genes regulated by adenomatous polyposis coli gene product (APC) and identified a novel gene, termed HELAD1 (helicase, APC down-regulated 1). A recombinant polypeptide representing the ATPases associated with cellular activities (AAA) domain of the HELAD1 product showed 3' to 5' helicase activity and exonuclease activity in vitro. HELAD1 was abundantly expressed in 16 of 20 colon cancers examined but hardly detectable in corresponding non-cancerous mucosae. Treatment of colon-cancer cells with antisense oligonucleotides suppressed its expression and induced apoptosis. These data revealed an importance of HELAD1 in colorectal carcinogenesis and suggest that suppression of HELAD1 may be a promising therapeutic strategy.

unc-53 controls longitudinal migration in C. elegans

Cell migration and outgrowth are thought to be based on analogous mechanisms that require repeated cycles of process extension, reading and integration of multiple directional signals, followed by stabilisation in a preferred direction, and renewed extension. We have characterised a C. elegans gene, unc-53, that appears to act cell autonomously in the migration and outgrowth of muscles, axons and excretory canals. Abrogation of unc-53 function disrupts anteroposterior outgrowth in those cells that normally express the gene. Conversely, overexpression of unc-53 in bodywall muscles leads to exaggerated outgrowth. UNC-53 is a novel protein conserved in vertebrates that contains putative SH3- and actin-binding sites. unc-53 interacts genetically with sem-5 and we demonstrated a direct interaction in vitro between UNC-53 and the SH2-SH3 adaptor protein SEM-5/GRB2. Thus, unc-53 is involved in longitudinal navigation and might act by linking extracellular guidance cues to the intracellular cytoskeleton.

Neuron navigator: a human gene family with homology to unc-53, a cell guidance gene from Caenorhabditis elegans

We have cloned the gene neuron navigator-1 (NAV1), a human homolog of unc-53, a gene involved in axon guidance in Caenorhabditis elegans. Duplications during evolution gave rise to three human homologs located on chromosomes 1q32.1, 11p15.1, and 12q21.1. NAV1 and NAV2 are expressed in the developing brain. NAV1, NAV2, and NAV3 expression is detected in adult heart, kidney, and brain, respectively. NAV1 encodes a protein lacking, in the aminoterminal part, a CH domain present in the other NAV genes. The first exon of NAV1 arose through an ancient internal duplication of sequences that also gave rise to exon 8 of NAV3 and exon 7 of NAV2. A detailed study of the NAV environment on the different chromosomes reveals incomplete micro-syntheny between the three regions. Through analysis of the phylogenetic relationships for three different gene families in the NAV environment, we reconstructed part of the events that formed these regions.

WH2 domain: a small, versatile adapter for actin monomers

The actin cytoskeleton plays a central role in many cell biological processes. The structure and dynamics of the actin cytoskeleton are regulated by numerous actin-binding proteins that usually contain one of the few known actin-binding motifs. WH2 domain (WASP homology domain-2) is a approximately 35 residue actin monomer-binding motif, that is found in many different regulators of the actin cytoskeleton, including the beta-thymosins, ciboulot, WASP (Wiskott Aldrich syndrome protein), verprolin/WIP (WASP-interacting protein), Srv2/CAP (adenylyl cyclase-associated protein) and several uncharacterized proteins. The most highly conserved residues in the WH2 domain are important in beta-thymosin's interactions with actin monomers, suggesting that all WH2 domains may interact with actin monomers through similar interfaces. Our sequence database searches did not reveal any WH2 domain-containing proteins in plants. However, we found three classes of these proteins: WASP, Srv2/CAP and verprolin/WIP in yeast and animals. This suggests that the WH2 domain is an ancient actin monomer-binding motif that existed before the divergence of fungal and animal lineages.

Functional plasticity of CH domains

With the refinement of algorithms for the identification of distinct motifs from sequence databases, especially those using secondary structure predictions, new protein modules have been determined in recent years. Calponin homology (CH) domains were identified in a variety of proteins ranging from actin cross-linking to signaling and have been proposed to function either as autonomous actin binding motifs or serve a regulatory function. Despite the overall structural conservation of the unique CH domain fold, the individual modules display a quite striking functional variability. Analysis of the actopaxin/parvin protein family suggests the existence of novel (type 4 and type 5) CH domain families which require special attention, as they appear to be a good example for how CH domains may function as scaffolds for other functional motifs of different properties.

Solution structure of the calponin CH domain and fitting to the 3D-helical reconstruction of F-actin:calponin

Calponin is involved in the regulation of contractility and organization of the actin cytoskeleton in smooth muscle cells. It is the archetypal member of the calponin homology (CH) domain family of actin binding proteins that includes cytoskeletal linkers such as alpha-actinin, spectrin, and dystrophin, and regulatory proteins including VAV, IQGAP, and calponin. We have determined the first structure of a CH domain from a single CH domain-containing protein, that of calponin, and have fitted the NMR-derived coordinates to the 3D-helical reconstruction of the F-actin:calponin complex using cryo-electron microscopy. The tertiary fold of this single CH domain is typical of, yet significantly different from, those of the CH domains that occur in tandem pairs to form high-affinity ABDs in other proteins. We thus provide a structural insight into the mode of interaction between F-actin and CH domain-containing proteins.

Towards a complete atomic structure of spectrin family proteins

The spectrin family of proteins represents a discrete group of cytoskeletal proteins comprising principally alpha-actinin, spectrin, dystrophin, and homologues and isoforms. They all share three main structural and functional motifs, namely, the spectrin repeat, EF-hands, and a CH domain-containing actin-binding domain. These proteins are variously involved in organisation of the actin cytoskeleton, membrane cytoskeleton architecture, cell adhesion, and contractile apparatus. The highly modular nature of these molecules has been a hindrance to the determination of their complete structures due to the inherent flexibility imparted on the proteins, but has also been an asset, inasmuch as the individual modules were of a size amenable to structural analysis by both crystallographic and NMR approaches. Representative structures of all the major domains shared by spectrin family proteins have now been solved at atomic resolution, including in some cases multiple domains from several family members. High-resolution structures, coupled with lower resolution methods to determine the overall molecular shape of these proteins, allow us for the first time to build complete atomic structures of the spectrin family of proteins.

The Abl-related gene (Arg) nonreceptor tyrosine kinase uses two F-actin-binding domains to bundle F-actin

Abl family nonreceptor tyrosine kinases regulate cellular morphogenesis and motility through functional interactions with the actin cytoskeleton. Although Abl family kinases are known to contain filamentous (F)-actin-binding domains at their C termini, it is unclear how Abl family kinases regulate the structure and/or function of the actin cytoskeleton. We show here that the Abl-related kinase Arg binds with positive cooperativity to F-actin in vitro with binding saturating at a ratio of one Arg/two actin molecules. Measurements of the F-actin-binding properties of Arg deletion mutants led to the identification of a second, previously uncharacterized internal F-actin-binding domain in Arg. Purified Arg can bundle F-actin in vitro, and this bundling activity requires both F-actin-binding domains. An Arg-yellow fluorescent protein fusion protein can induce the formation of actin-rich structures at the lamellipodia of Swiss 3T3 fibroblasts. Both of Arg's F-actin-binding domains are necessary and sufficient for the formation of these actin-rich structures. Together, our data suggest that Arg can use its F-actin-bundling activity to directly regulate actin cytoskeletal structure in vivo.

β1PIX, the PAK-interacting exchange factor, requires localization via a coiled-coil region to promote microvillus-like structures and membrane ruffles

PIX is a Rho-family guanine nucleotide exchange factor that binds PAK. We previously described two isoforms of PIX that differ in their N termini. Here, we report the identification of a new splice variant of βPIX, designated β2PIX, that is the dominant species in brain and that lacks the region of ∼120 residues with predicted coiled-coil structure at the C terminus of β1PIX. Instead, β2PIX contains a serine-rich C terminus. To determine whether these splice variants differ in their cellular function, we studied the effect of expressing these proteins in HeLa cells. We found that the coiled-coil region plays a key role in the localization of β1PIX to the cell periphery and is also responsible for PIX dimerization. Overexpression of β1, but not β2PIX, drives formation of membrane ruffles and microvillus-like structures (via activation of Rac1 and Cdc42, respectively), indicating that its function requires localized activation of these GTPases. Thus, β1PIX, like other RhoGEFs, exerts specific morphological functions that are dependent on its intracellular location and are mediated by its C-terminal dimerization domain.

A novel integrin-linked kinase-binding protein, affixin, is involved in the early stage of cell-substrate interaction

Focal adhesions (FAs) are essential structures for cell adhesion, migration, and morphogenesis. Integrin-linked kinase (ILK), which is capable of interacting with the cytoplasmic domain of beta1 integrin, seems to be a key component of FAs, but its exact role in cell-substrate interaction remains to be clarified. Here, we identified a novel ILK-binding protein, affixin, that consists of two tandem calponin homology domains. In CHOcells, affixin and ILK colocalize at FAs and at the tip of the leading edge, whereas in skeletal muscle cells they colocalize at the sarcolemma where cells attach to the basal lamina, showing a striped pattern corresponding to cytoplasmic Z-band striation. When CHO cells are replated on fibronectin, affixin and ILK but not FA kinase and vinculin concentrate at the cell surface in blebs during the early stages of cell spreading, which will grow into membrane ruffles on lamellipodia. Overexpression of the COOH-terminal region of affixin, which is phosphorylated by ILK in vitro, blocks cell spreading at the initial stage, presumably by interfering with the formation of FAs and stress fibers. The coexpression of ILK enhances this effect. These results provide evidence suggesting that affixin is involved in integrin-ILK signaling required for the establishment of cell-substrate adhesion.

Calmin, a protein with calponin homology and transmembrane domains expressed in maturing spermatogenic cells

A cDNA named calmin of approximately 3.2 kb was isolated by RNA differential display applied to developing mouse skin. Calmin cDNA encodes 1021 amino acids with two calponin homology (CH) domains in tandem on the N-terminal side and a transmembrane domain on the C-terminal side. The region covering the CH domains showed a high level of homology with beta-spectrin, alpha-actinin, and dystrophin. Among the proteins with the tandem CH domains, calmin is unique in having a transmembrane domain. Three alternative splicing sites were identified at the 3'-side of calmin, giving rise to polymorphic protein products with or without the transmembrane domain. The calmin transcript was detected in adult testis, liver, kidney, and large intestine; the expression in testis was far stronger than that in the other tissues. In situ hybridization and immunostaining revealed that calmin was expressed in maturing spermatogenic cells at later stages. Human calmin cDNA was also isolated, and its exon/intron organization was determined.

Binding of dystrophin's tandem calponin homology domain to F-actin is modulated by actin's structure

Dystrophin has been shown to be associated in cells with actin bundles. Dys-246, an N-terminal recombinant protein encoding the first 246 residues of dystrophin, includes two calponin-homology (CH) domains, and is similar to a large class of F-actin cross-linking proteins including alpha-actinin, fimbrin, and spectrin. It has been shown that expression or microinjection of amino-terminal fragments of dystrophin or the closely related utrophin resulted in the localization of these protein domains to actin bundles. However, in vitro studies have failed to detect any bundling of actin by either intact dystrophin or Dys-246. We show here that the structure of F-actin can be modulated so that there are two modes of Dys-246 binding, from bundling actin filaments to only binding to single filaments. The changes in F-actin structure that allow Dys-246 to bundle filaments are induced by covalent modification of Cys-374, proteolytic cleavage of F-actin's C-terminus, mutation of yeast actin's N-terminus, and different buffers. The present results suggest that F-actin's structural state can have a large influence on the nature of actin's interaction with other proteins, and these different states need to be considered when conducting in vitro assays.

UNC-87 is an actin-bundling protein

The Caenorhabditis elegans unc-87 gene product is essential for the maintenance of the nematode body wall muscle where it is found colocalized with actin in the I band. The molecular domain structure of the protein reveals similarity to the C-terminal repeat region of the smooth muscle actin-binding protein calponin. In this study we investigated the in vitro function of UNC-87 using both the full-length recombinant molecule and several truncated mutants. According to analytical ultracentrifugation UNC-87 occurs as a monomer in solution. UNC-87 cosedimented with both smooth and skeletal muscle F-actin, but not with monomeric G-actin, and exhibited potent actin filament bundling activity. Actin binding was independent of the presence of tropomyosin and the actin cross-linking proteins filamin and alpha-actinin. Consistent with its actin bundling activity in vitro, UNC-87 tagged with green fluorescent protein associated with and promoted the formation of actin stress fiber bundles in living cells. These data identify UNC-87 as an actin-bundling protein and highlight the calponin-like repeats as a novel actin-binding module.

Parvin, a 42 kDa focal adhesion protein, related to the alpha-actinin superfamily

We have identified and cloned a novel 42-kDa protein termed alpha-parvin, which has a single alpha-actinin-like actin-binding domain. Unlike other members of the alpha-actinin superfamily, which are large multidomain proteins, alpha-parvin lacks a rod domain or any other C-terminal structural modules and therefore represents the smallest known protein of the superfamily. We demonstrate that mouse alpha-parvin is widely expressed as two mRNA species generated by alternative use of two polyadenylation signals. We analyzed the actin-binding properties of mouse alpha-parvin and determined the K(d) with muscle F-actin to be 8.4+/-2.1 microM. The GFP-tagged alpha-parvin co-localizes with actin filaments at membrane ruffles, focal contacts and tensin-rich fibers in the central area of fibroblasts. Domain analysis identifies the second calponin homology domain of parvin as a module sufficient for targeting the focal contacts. In man and mouse, a closely related paralogue beta-parvin and a more distant relative gamma-parvin have also been identified and cloned. The availability of the genomic sequences of different organisms enabled us to recognize closely related parvin-like proteins in flies and worms, but not in yeast and Dictyostelium. Phylogenetic analysis of alpha-parvin and its para- and orthologues suggests, that the parvins represent a new family of alpha-actinin-related proteins that mediate cell-matrix adhesion.

Mutual effects of alpha-actinin, calponin and filamin on actin binding

The mutual effect of three actin-binding proteins (alpha-actinin, calponin and filamin) on the binding to actin was analyzed by means of differential centrifugation and electron microscopy. In the absence of actin alpha-actinin, calponin and filamin do not interact with each other. Calponin and filamin do not interfere with each other in the binding to actin bundles. Slight interference was observed in the binding of alpha-actinin and calponin to actin bundles. Higher ability of calponin to depress alpha-actinin binding can be due to the higher stoichiometry calponin/actin in the complexes formed. The largest interference was observed in the pair filamin-alpha-actinin. These proteins interfere with each other in the binding to the bundled actin filaments; however, neither of them completely displaced another protein from its complexes with actin. The structure of actin bundles formed in the presence of any one actin-binding protein was different from that observed in the presence of binary mixtures of two actin-binding proteins. In the case of calponin or its binary mixtures with alpha-actinin or filamin the total stoichiometry actin-binding protein/actin was larger than 0.5. This means that alpha-actinin, calponin and filamin may coexist on actin filaments and more than mol of any actin-binding protein is bound per two actin monomers. This may be important for formation of different elements of cytoskeleton.

Actopaxin, a new focal adhesion protein that binds paxillin LD motifs and actin and regulates cell adhesion

Paxillin is a focal adhesion adapter protein involved in the integration of growth factor- and adhesion-mediated signal transduction pathways. Paxillin LD motifs have been demonstrated to bind to several proteins associated with remodeling of the actin cytoskeleton including the focal adhesion kinase, vinculin, and a complex of proteins comprising p95PKL, PIX, and PAK (Turner, C.E., M. C. Brown, J.A. Perrotta, M.C. Riedy, S.N. Nikolopoulos, A.R. McDonald, S. Bagrodia, S. Thomas, and P.S. Leventhal. 1999. J. Cell Biol. 145:851-863). In this study, we report the cloning and initial characterization of a new paxillin LD motif-binding protein, actopaxin. Analysis of the deduced amino acid sequence of actopaxin reveals a 42-kD protein with two calponin homology domains and a paxillin-binding subdomain (PBS). Western blotting identifies actopaxin as a widely expressed protein. Actopaxin binds directly to both F-actin and paxillin LD1 and LD4 motifs. It exhibits robust focal adhesion localization in several cultured cell types but is not found along the length of the associated actin-rich stress fibers. Similar to paxillin, it is absent from actin-rich cell-cell adherens junctions. Also, actopaxin colocalizes with paxillin to rudimentary focal complexes at the leading edge of migrating cells. An actopaxin PBS mutant incapable of binding paxillin in vitro cannot target to focal adhesions when expressed in fibroblasts. In addition, ectopic expression of the PBS mutant and/or the COOH terminus of actopaxin in HeLa cells resulted in substantial reduction in adhesion to collagen. Together, these results suggest an important role for actopaxin in integrin-dependent remodeling of the actin cytoskeleton during cell motility and cell adhesion.

Biochemical characterisation of the actin-binding properties of utrophin

Utrophin is a large ubiquitously expressed cytoskeletal protein that is important for maturation of vertebrate neuromuscular junctions. It is highly homologous to dystrophin, the protein defective in Duchenne and Becker muscular dystrophies. Utrophin binds to the actin cytoskeleton via an N-terminal actin-binding domain, which is related to the actin-binding domains of members of the spectrin superfamily of proteins. We have determined the actin-binding properties of this utrophin domain and investigated its binding site on F-actin. An F-actin cosedimentation assay confirmed that the domain binds more tightly to beta-F-actin than to alpha-F-actin and that the full-length utrophin domain binds more tightly to both actin isoforms than a truncated construct, lacking a characteristic utrophin N-terminal extension. Both domain constructs exist in solution as compact monomers and bind to actin as 1:1 complexes. Analysis of the products of partial proteolysis of the domain in the presence of F-actin showed that the N-terminal extension was protected by binding to actin. The actin isoform dependence of utrophin binding could reflect differences at the N-termini of the actin isoforms, thus localising the utrophin-binding site on actin. The involvement of the actin N-terminus in utrophin binding was also supported by competition binding assays using myosin subfragment S1, which also binds F-actin near its N-terminus. Cross-linking studies suggested that utrophin contacts two actin monomers in the actin filament as does myosin S1. These biochemical approaches complement our structural studies and facilitate characterisation of the actin-binding properties of the utrophin actin-binding domain.

The structure of the N-terminal actin-binding domain of human dystrophin and how mutations in this domain may cause Duchenne or Becker muscular dystrophy

BACKGROUND

Dystrophin is an essential component of skeletal muscle cells. Its N-terminal domain binds to F-actin and its C terminus binds to the dystrophin-associated glycoprotein (DAG) complex in the membrane. Dystrophin is therefore thought to serve as a link from the actin-based cytoskeleton of the muscle cell through the plasma membrane to the extracellular matrix. Pathogenic mutations in dystrophin result in Duchenne or Becker muscular dystrophy.

RESULTS

The crystal structure of the dystrophin actin-binding domain (ABD) has been determined at 2.6 A resolution. The structure is an antiparallel dimer of two ABDs each comprising two calponin homology domains (CH1 and CH2) that are linked by a central alpha helix. The CH domains are both alpha-helical globular folds. Comparisons with the structures of utrophin and fimbrin ABDs reveal that the conformations of the individual CH domains are very similar to those of dystrophin but that the arrangement of the two CH domains within the ABD is altered. The dystrophin dimer reveals a change of 72 degrees in the orientation of one pair of CH1 and CH2 domains (from different monomers) relative to the other pair when compared with the utrophin dimer. The dystrophin monomer is more elongated than the fimbrin ABD.

CONCLUSIONS

The dystrophin ABD structure reveals a previously uncharacterised arrangement of the CH domains within the ABD. This observation has implications for the mechanism of actin binding by dystrophin and related proteins. Examining the position of three pathogenic missense mutations within the structure suggests that they exert their effects through misfolding of the ABD, rather than through disruption of the binding to F-actin.

Specific subdomains of Vav differentially affect T cell and NK cell activation

The Vav protooncogene is a multidomain protein involved in the regulation of IL-2 gene transcription in T cells and the development of cell-mediated killing by cytotoxic lymphocytes. We have investigated the differential roles that specific protein subdomains within the Vav protooncogene have in the development of these two distinct cellular processes. Interestingly, a calponin homology (CH) domain mutant of Vav (CH-) fails to enhance NF-AT/AP-1-mediated gene transcription but is still able to regulate the development of cell-mediated killing. The inability of the CH- mutant to enhance NF-AT/AP-1-mediated transcription appears to be secondary to defective intracellular calcium, because 1) the CH- mutant has significantly reduced TCR-initiated calcium signaling, and 2) treatment with the calcium ionophore ionomycin or cotransfection with activated calcineurin restores NF-AT/AP-1-mediated gene transcription. The pleckstrin homology (PH) domain of Vav has also been implicated in regulating Vav activation. We found that deletion of the PH domain of Vav yields a protein that can neither enhance gene transcription from the NF-AT/AP-1 reporter nor enhance TCR- or FcR-mediated killing. In contrast, the PH deletion mutant of Vav is able to regulate the development of natural cytotoxicity, indicating a functional dichotomy for the PH domain in the regulation of these two distinct forms of killing. Lastly, mutation of three tyrosines (Y142, Y160, and Y174) within the acidic domain of Vav has revealed a potential negative regulatory site. Replacement of all three tyrosines with phenylalanine results in a hyperactive protein that increases NF-AT/AP-1-mediated gene transcription and enhances cell-mediated cytotoxicity. Taken together, these data highlight the differential roles that specific subdomains of Vav have in controlling distinct cellular functions. More broadly, the data suggest that separate lymphocyte functions can potentially be modulated by domain-specific targeting of Vav and other critical intracellular signaling molecules.

Identification and characterisation of transcript and protein of a new short N-terminal utrophin isoform

Dystrophin and utrophin are known to link the intracellular cytoskeleton to the extracellular matrix via a transmembraneous glycoprotein complex. Four short C-terminal isoforms (Dp71, Dp116, Dp140, and Dp260) are described for dystrophin and three for utrophin (Up71, Up113, and Up140). We describe here for the first time the existence of a 3.7-kb transcript and a 62-kDa protein in C6 glioma cells representing a short N-terminal isoform unique for utrophin (N-utrophin). More than 20 clones covering the entire coding region of utrophin were isolated from a rat C6 glioma cell cDNA library. Two clones were found to code for a protein with 539 amino acids. Its sequence is identical to that of the full-length utrophin, except for the last residue where Cys is replaced by Val. This isoform contains the actin binding domain (consisting of two calponin homology subdomains), followed by two spectrin-like repeats. A recombinant fragment corresponding to N-utrophin binds to F-actin in vitro with an equilibrium constant (affinity) K of 4.5 x 10(5) M(-1) and a stoichiometry of one fragment per around five actin monomers. Immunocytochemical staining of C6 glioma cells with antisera specific for different utrophin regions localised full-length utrophin in the submembraneous cortical actin layer as revealed by confocal microscopy. A distinct staining pattern for the N-utrophin was not detectable, although it was expected to localise at the actin stress fibers. It is assumed that it co-localises via the two spectrin-like repeats with the full-length utrophin at the cell membrane.

Structure of the utrophin actin-binding domain bound to F-actin reveals binding by an induced fit mechanism

Utrophin is a large ubiquitously expressed cytoskeletal protein, homologous to dystrophin, the protein disrupted in Duchenne muscular dystrophy. The association of both proteins with the actin cytoskeleton is functionally important and is mediated by a domain at their N termini, conserved in members of the spectrin superfamily, including alpha-actinin, beta-spectrin and fimbrin. We present the structure of the actin-binding domain of utrophin in complex with F-actin, determined by cryo-electron microscopy and helical reconstruction, and a pseudo-atomic model of the complex, generated by docking the crystal structures of the utrophin domain and F-actin into the reconstruction. In contrast to the model of actin binding proposed for fimbrin, the utrophin actin-binding domain appears to associate with actin in an extended conformation. This conformation places residues that are highly conserved in utrophin and other members of the spectrin superfamily at the utrophin interface with actin, confirming the likelihood of this binding orientation. This model emphasises the importance of protein flexibility in modeling interactions and presents the fascinating possibility of a diversity of actin-binding mechanisms among related proteins.

Heat shock protein (hsp90) interacts with smooth muscle calponin and affects calponin-binding to actin

Interaction of smooth muscle calponin with 90 kDa heat shock protein (hsp90) was analyzed by means of native gel electrophoresis and affinity chromatography. Under conditions used, calponin and hsp90 form a complex with an apparent dissociation constant in the micromolar range. The major hsp90-binding site is located in the N-terminal (residues 7-144) part of calponin. Addition of calponin to actin-tropomyosin complex results in formation of actin bundles. Hsp90 partially prevents bundle formation without affecting the molar ratio calponin/actin in single actin filaments or actin bundles. At low ionic strength, calponin induces polymerization of G-actin. Hsp90 decreases calponin-induced polymerization of G-actin. It is supposed that hsp90 may be involved in the assembly of actin filaments.

Actin and the smooth muscle regulatory proteins: a structural perspective

The structural details of the smooth muscle acto-myosin interaction and its functional implications have been much discussed in recent years, however other, smooth muscle specific, actin-binding proteins have received much less attention. With increasing technical advances in structural biology a great deal of structural information is now coming to light, information that can provide useful insight into the mechanism of action for many important nonmotor actin-binding proteins. The purpose of the review is to instill the current knowledge on the structure, and interaction sites on F-actin, of the major, non-motor actin-binding proteins from smooth muscle, proposed to have a role in regulation. In the light of the recent structural studies the probable roles of the various actin-binding proteins will be discussed with particular reference to structure function relationships.

Crystal structure of the actin-binding region of utrophin reveals a head-to-tail dimer

BACKGROUND

Utrophin is a large multidomain protein that belongs to a superfamily of actin-binding proteins, which includes dystrophin, alpha-actinin, beta-spectrin, fimbrin, filamin and plectin. All the members of this family contain a common actin-binding region at their N termini and perform a wide variety of roles associated with the actin cytoskeleton. Utrophin is the autosomal homologue of dystrophin, the protein defective in the X-linked Duchenne and Becker muscular dystrophies, and upregulation of utrophin has been suggested as a potential therapy for muscular dystrophy patients.

RESULTS

The structure of the actin-binding region of utrophin, consisting of two calponin-homology (CH) domains, has been solved at 3.0 A resolution. It is composed of an antiparallel dimer with each of the monomers being present in an extended dumbell shape and the two CH domains being separated by a long central helix. This extended conformation is in sharp contrast to the compact monomer structure of the N-terminal actin-binding region of fimbrin.

CONCLUSIONS

The crystal structure of the actin-binding region of utrophin suggests that these actin-binding domains may be more flexible than was previously thought and that this flexibility may allow domain reorganisation and play a role in the actin-binding mechanism. Thus utrophin could possibly bind to actin in an extended conformation so that the sites previously identified as being important for actin binding may be directly involved in this interaction.

Involvement of NH(2)-terminal sequences in the negative regulation of Vav signaling and transforming activity

Deletion of the NH(2)-terminal 65 amino acids of proto-Vav (to form onco-Vav) activates its transforming activity, suggesting that these sequences serve a negative regulatory role in Vav function. However, the precise role of these NH(2)-terminal sequences and whether additional NH(2)-terminal sequences are also involved in negative regulation have not been determined. Therefore, we generated additional NH(2)-terminal deletion mutants of proto-Vav that lack the NH(2)-terminal 127, 168, or 186 amino acids, and assessed their abilities to cause focus formation in NIH 3T3 cells and to activate different signaling pathways. Since Vav mutants lacking 168 or 186 NH(2)-terminal residues showed a several 100-fold greater focus forming activity than that seen with deletion of 65 residues, residues spanning 66 to 187 also contribute significantly to negative regulation of Vav transforming activity. The increase in Vav transforming activity correlated with the activation of the c-Jun, Elk-1, and NF-kappaB transcription factors, as well as increased transcription from the cyclin D1 promoter. Tyrosine 174 is a key site of phosphorylation by Lck in vitro and Lck-mediated phosphorylation has been shown to be essential for proto-Vav GEF function in vitro. However, we found that an NH(2)-terminal Vav deletion mutant lacking this tyrosine residue (DeltaN-186 Vav) retained the ability to be phosphorylated by Lck in vivo and Lck still caused enhancement of DeltaN-186 Vav signaling and transforming activity. Thus, Lck can stimulate Vav via a mechanism that does not involve Tyr(174) or removal of NH(2)-terminal regulatory activity. Finally, we found that NH(2)-terminal deletion enhanced the degree of Vav association with the membrane-containing particulate fraction and that an isolated NH(2)-terminal fragment (residues 1-186) could impair DeltaN-186 Vav signaling. Taken together, these observations suggest that the NH(2) terminus may serve as a negative regulator of Vav by intramolecular interaction with COOH-terminal sequences to modulate efficient membrane association.

N-terminally truncated Vav induces the formation of depolymerization-resistant actin filaments in NIH 3T3 cells

The Dbl family proto-oncogene vav is a guanine nucleotide exchange factor (GEF) for Rho family GTPases. Deletion of the N-terminus of Vav, harboring the single calponin homology (CH) domain, activates Vav's transforming potential, suggesting an important role of the CH domain in influencing Vav function. Since calponin binds actin, it has been suggested that the CH domain may mediate association with the actin cytoskeleton. In this study we have analyzed the subcellular localization and investigated the putative actin association of the Vav protein using enhanced green fluorescent protein (EGFP) fusion constructs. Our data show that both EGFP-tagged full length Vav and the CH domain-depleted EGFPvav 143-845 construct localize throughout the cytoplasm but fail to colocalize with F-actin. However, the latter construct of Vav was more strongly retained in the Triton-insoluble cytoskeleton fraction than full length Vav. Whereas removal of the CH domain had no apparent influence on the subcellular localization of Vav, deletion of the SH domains caused nuclear localization, indicating that Vav contains a functional nuclear localization signal. Expression of N-terminally truncated Vav constructs caused depolarization of fibroblasts and triggered the bundling of actin stress fibers into parallel arrays in NIH 3T3 cells. Notably, the parallel actin bundles showed prolonged resistance to the actin polymerization antagonists cytochalasin B and latrunculin B. These data point towards a regulatory role for the CH domain in Vav and suggest an actin cross-linking or bundling protein as a downstream effector molecule of vav-mediated signalling pathways.

Brains and brawn: plectin as regulator and reinforcer of the cytoskeleton

Plectin is a 580 kDa intracellular protein, previously shown to link intermediate filaments with microtubules, actin filaments, and membrane components. Disruption of the plectin gene in humans and in mice results in severe skin blistering and muscular degeneration, consistent with plectin's structural role in stabilizing cells against mechanical force. However, recent work by Andra et al. characterizing cells from plectin-deficient mice demonstrates that in addition to this structural role, plectin also modulates the dynamics of the actin cytoskeleton. This makes plectin unusual in that it serves both to reinforce and crosslink intermediate filament attachments to membranes and other cytoskeletal polymers and to regulate actin dynamics in cells.

Plectin: a cytolinker by design

Plectin is a cytoskeletal protein of >500 kDa that forms dumbbell-shaped homodimers comprising a central parallel alpha-helical coiled coil rod domain flanked by globular domains, thus providing a molecular backbone ideally suited to mediate the protein's interactions with an array of other cytoskeletal elements. Plectin self-associates and interacts with actin and intermediate filament cytoskeleton networks at opposite ends, and it binds at both ends to the hemidesmosomal transmembrane protein integrin beta-4, and likely to other junctional proteins. The central coiled coil rod domain can form bridges over long stretches and serves as a flexible linker between the structurally diverse N-terminal domain and the highly conserved C-terminal domain. Plectin is also a target of p34cdc2 kinase that regulates its dissociation from intermediate filaments during mitosis.

The 2.0 A structure of the second calponin homology domain from the actin-binding region of the dystrophin homologue utrophin

Utrophin is a close homologue of dystrophin, the protein defective in Duchenne muscular dystrophy. Like dystrophin, it is composed of three regions: an N-terminal region that binds actin filaments, a large central region with triple coiled-coil repeats, and a C-terminal region that interacts with components in the dystroglycan protein complex at the plasma membrane. The N-terminal actin-binding region consists of two calponin homology domains and is related to the actin-binding domains of a superfamily of proteins including alpha-actinin, spectrin and fimbrin. Here, we present the 2.0 A structure of the second calponin homology domain of utrophin solved by X-ray crystallography, and compare it to the other calponin homology domains previously determined from spectrin and fimbrin.

Regulation of smooth muscle actin-myosin interaction and force by calponin

Smooth muscle contraction is regulated primarily by the reversible phosphorylation of myosin triggered by an increase in sarcoplasmic free Ca2+ concentration ([Ca2+]i). Contraction can, however, be modulated by other signal transduction pathways, one of which involves the thin filament-associated protein calponin. The h1 (basic) isoform of calponin binds to actin with high affinity and is expressed specifically in smooth muscle at a molar ratio to actin of 1:7. Calponin inhibits (i) the actin-activated MgATPase activity of smooth muscle myosin (the cross-bridge cycling rate) via its interaction with actin, (ii) the movement of actin filaments over immobilized myosin in the in vitro motility assay, and (iii) force development or shortening velocity in permeabilized smooth muscle strips and single cells. These inhibitory effects of calponin can be alleviated by protein kinase C (PKC)-catalysed phosphorylation and restored following dephosphorylation by a type 2A phosphatase. Three physiological roles of calponin can be considered based on its in vitro functional properties: (i) maintenance of relaxation at resting [Ca2+]i, (ii) energy conservation during prolonged contractions, and (iii) Ca(2+)-independent contraction mediated by phosphorylation of calponin by PKC epsilon, a Ca(2+)-independent isoenzyme of PKC.

Structural comparisons of calponin homology domains: implications for actin binding

BACKGROUND

The actin-binding site of several cytoskeletal proteins is comprised of two calponin homology (CH) domains in a tandem arrangement. As a single copy, the CH domain is also found in regulatory proteins in muscle and in signal-transduction proteins. The three-dimensional structures of three CH domains are known, but they have not yet clarified the molecular details of the interaction between actin filaments and proteins harbouring CH domains.

RESULTS

We have compared the crystal structure of a CH domain from beta-spectrin, which has been refined to 1.1 A resolution, with the two CH domains that constitute the actin-binding region of fimbrin. This analysis has allowed the construction of a structure-based sequence alignment of CH domains that can be used in further comparisons of members of the CH domain family. The study has also improved our understanding of the factors that determine domain architecture, and has led to discussion on the functional differences that seem to exist between subfamilies of CH domains, as regards binding to F-actin.

CONCLUSIONS

Our analysis supports biochemical data that implicate a surface centered at the last helix of the N-terminal CH domain as the most probable actin-binding site in cytoskeletal proteins. It is not clear whether the C-terminal domains of the tandem arrangement or the single CH domains have this function alone. This may imply that although the CH domains are homologous and have a conserved structure, they may have evolved to perform different functions.